1. Field of the Invention
This invention relates to a method and an apparatus for enhancing reception in a mobile broadcast band FM receiver and more particularly to method and apparatus which affords continuous selection among two or more received samples of a transmitted signal.
2. Description of the Prior Art
FM broadcast utilizes transmissions in the VHF range which typically traverse a line of sight path. Receivers disposed in fringe areas or other locations without a line of sight path to the transmitter often receive plural signals which arrive at the receiver via different paths due to diffraction, refraction and/or reflection. The condition is known as multipath reception. Where plural signals arriving at the receiver are out of phase, the signals can partially or completely cancel one another and significantly degrade reception quality. A known expedient for reducing the adverse effects of signal cancellation due to out of phase arrival of the transmitted signals is to provide two antennas at spaced apart locations and/or of different polarizations and to connect the antenna having the strongest signal to the receiver. This is called diversity reception; the benefit accrues because the momentary multipath disturbances may not occur simultaneously at the two antennas.
Exemplifying such known expedient is the system disclosed in U.S. Pat. No. 2,729,741 in which one of two antennas is connected via a relay switch to a receiver. The relay switch is controlled by a circuit which measures the detected or demodulated receiver output arising from each of the two antenna signals and operates the switch so as to connect the antenna with the stronger signal to the receiver. The system continuously tests the signals received by two antennas and switches the receiver to the antenna receiving the stronger signal even though the signal received by the other antenna is of adequate strength. Such switching takes place at a relatively slow rate, a rate within the audible range, and therefore, is discernible by and bothersome to the listener, particularly when the program material is wide band stereophonic sound as typically broadcast in the FM band.
U.S. Pat. No. 2,872,568 discloses a similar system in which one of two antennas is connected to a receiver by means of a twin triode switch. As in the case of the system disclosed in the U.S. Pat. No. 2,729,741, the circuitry of patent 2,872,568 responds to the detected or demodulated signal as a result of which the switching of the antennas is audible to the listener of the receiver with which the patented system is employed. Moreover, the listener frequently hears a signal fade before the patented system switches to the other antenna.
In an article titled "FM Multipath Distortion in Automobile Receivers Has Been Significantly Reduced by a New Antenna System" by Takeda et al, IEEE Transactions on Consumer Electronics, vol. CE-26, Aug. 1980, there is described a system employing two orthogonally disposed loop antennas and a switching circuit which couples the antenna having the better received signal to a mobile FM receiver. The sensing circuit disclosed in the article responds to the strength of the signal only after it has been demodulated or detected in consequence of which system operation is substantially influenced by the nature of the program material and the selective fading of the material.
The previously described prior art references as well as the present invention can be better understood by considering the phenomena that adversely affect the quality of the received signal in a mobile FM broadcast band receiver; such phenomena will be briefly described.
"Fast fading" occurs within a fraction of a wavelength due to signal reflections from obstacles near the vehicle in which the receiver is mounted. A vehicle moving through regions where such reflections are present experiences fades at a rate dependent upon vehicle speed.
"Slow fading" occurs over tens of wavelengths and is associated with terrain profile and the general nature of the environment, which influence the relative phase difference of the signals arriving at the receiver antennas.
"Selective fading" refers to fading that varies in degree with the frequency of the signal modulated onto the carrier. Selective fading is important in FM broadcast reception because in the case of stereophonic material the modulation has a wide band width, typically 150 KHz. In some areas, lower modulation frequencies are typically received without difficulty, but selective fading can occur with full program modulation, i.e. modulation containing relatively high frequency signals. Selective fading can occur whether or not the vehicle carrying the receiver is moving and can adversely affect reception within microseconds.
All of the foregoing fading phenomena arise at least in part from multipath signals arriving at an antenna. Consequently, the adverse effects arising from the phenomena can be ameliorated by diversity reception employing two antennas that are physically spaced apart from one another or that are polarized differently from one another so that disturbing fades are not simultaneous.
The program material, the information, appears in the transmission as a frequency modulation; signal amplitude is nominally constant in the short term. The multipath disturbances appear typically as superimposed amplitude fluctuations with characteristics in accord with fade type. While an FM receiver is designed to discriminate against momentary noise impulses (as from lightning), the multipath phenomena may modify the character of the FM transmission to create severe disturbances in the forms of noise and distortion in the program material.
The success of diversity reception as an ameliorating factor depends on obtaining signal samples from antennas independently affected by multipath, then providing circuitry for deciding which antenna should be connected at any moment and switching to that antenna such that switching transients do not, themselves, impair the quality of reception. This invention is directed to such circuitry.